Refactor layout writing to avoid slice creation

[jphamata]

Summary

This PR refactors FFI functions (qk_transpile_layout_initial_layout, qk_transpile_layout_output_permutation and qk_transpile_layout_final_layout) in transpile_layout.rs to avoid creating mutable references from raw pointers. The previous implementation used from_raw_parts_mut, which is not sound when the pointed memory may be uninitialized. This PR switches to ptr.add(i).write(...) loops.

Details and comments

Fixes #15370

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[debasmita2102]

Thanks for the refactor improving memory safety in layout writing. I noticed there are no new or modified tests in this PR related to these changes. Could you please confirm if existing tests fully cover the affected code paths? If not, are there plans to add tests to verify these memory safety improvements and prevent regressions? This would help ensure the robustness of this fix.

[jphamata]

Hi! Just to update, all existing tests that cover transpile_layout.rs passed (tox -e rust), confirming that the refactor didn’t change behavior. I also ran Miri, and the only undefined behavior it detected comes from another module (neighbors.rs), not from the code modified in this PR. So I believe the current coverage is sufficient, but I can look into adding a specific test if needed.

[coveralls]

Pull Request Test Coverage Report for Build 19693981737

Warning: This coverage report may be inaccurate.

This pull request's base commit is no longer the HEAD commit of its target branch. This means it includes changes from outside the original pull request, including, potentially, unrelated coverage changes.

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• For a quick fix, rebase this PR at GitHub. Your next report should be accurate.

Details

• 12 of 12 (100.0%) changed or added relevant lines in 1 file are covered.
• 10 unchanged lines in 3 files lost coverage.
• Overall coverage increased (+0.005%) to 88.251%

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Files with Coverage Reduction	New Missed Lines	%
crates/transpiler/src/passes/unitary_synthesis.rs	1	92.57%
crates/qasm2/src/lex.rs	4	92.8%
crates/circuit/src/parameter/symbol_expr.rs	5	73.19%

Totals
Change from base Build 19684338744:	0.005%
Covered Lines:	94406
Relevant Lines:	106975

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💛 - Coveralls